1. Field of the Invention
The present invention relates to a process for the production of a mixed or composite carbide for a cemented carbide alloy, in particular, in which part of the tungsten carbide in the alloy is replaced by molybdenum carbide and more particularly, it is concerned with a process for the production of a mixed or composite carbide consisting mainly of molybdenum carbide and tungsten carbide in the form of a solid solution whose crystal structure is of a simple hexagonal type.
2. Description of the Prior Art
Up to the present time, as a starting material for cemented carbides, there has been used tungsten carbide (WC) powder as a predominant component with a suitable binder metal, typically an iron group metal, to which carbides or carbonitrides of high melting point metals such as titanium, tantalum, niobium, molybdenum, hafnium, vanadium and chromium are added depending on the requirements of a desired alloy. However, it is also true that tungsten is a relatively expensive metal and that it is found in only a few parts of the world. Accordingly, it is considered to be a so-called "strategic" material, and its availability can be subject to political considerations. Therefore, increase of the demand for cemented carbides consisting mainly of tungsten carbide meets inevitably with a problem of natural resources, and if the tungsten carbide can be exchanged for another high melting point metal carbide, this exchange has a great influence upon the industry.
Molybdenum monocarbide (MoC) is considered as a useful substitute, since this carbide only has the same crystal structure of simple hexagonal type as tungsten carbide, as well as mechanical properties similar to tungsten carbide. However, the existence of the hexagonal molybdenum monocarbide as a simple substance has remained in question to this date and thus an attempt to stabilize molybdenum has exclusively been carried out by forming a solid solution with tungsten carbide. This method was first reported by W. Dawihl in 1950, but this solid solution was not examined in detail and its commercial worth was not found in those days.
Of late, however, the study to utilize the solid solution (Mo.sub.x W.sub.y)C where x+y=1 has become active with the rise of the price of tungsten. It is very interesting why a study on this solid solution and an attempt to use the same have not been carried out so actively.
In the prior art process for the production of a solid solution of MoC-WC, WC, Mo and C powders or W, Mo, C and Co powders are mixed, charged in a carbon vessel and reacted at a temperature of 1600.degree. to 2000.degree. C. (W. Dawihl; "Zeitschrift f. Anorganische Chemie" 262 (1950) 212). In this case, cobalt serves to aid in forming the carbide and to dissolve molybdenum and carbon in the tungsten carbide. In the absence of cobalt, it is very difficult to obtain a solid solution of (Mo, W)C. When a (Mo, W)C powder obtained by this method is used for the production of a cemented carbide alloy with a binder metal of Co as a substitute for WC, however, (Mo, W)C decomposes in the alloy to deposit needle crystals of (Mo, W).sub.2 C. Deposition of even a small amount of such a subcarbide with aggregation in the alloy causes deterioration of the strength of the alloy. For this reason, the use of MoC as a substitute for WC has not been attempted positively.